Titanium dioxide, which is n-type semiconductor, has been known as a photocatalyst that activates various chemical reactions with ultraviolet radiation energy, such as chemical reactions resulted in during a process of decomposition of water, deodorization, pateurization, cleaning of water, a treatment of waste water or the like. It is said that the catalytic activity of a photocatalyst can be generally high when it is used either in powder form or in a form of suspension in a solvent, however, in many cases, such photocatalyst is practically obliged to be used in a form being carried on a certain substrate. For utilizing ultraviolet radiation energy from light efficiently, it is advantageous to make a substrate into a shape like a paper or a sheet, which can secure the wider light irradiation area, moreover, it is further advantageous to make the surface of the substrate into a porous structure in order to increase contacting area of the substrate with a reactant with which an objective chemical reaction is desired to be proceeded in the presence of a photocatalyst.
Various substrates which comprise a photocatalyst have been proposed in the past. For example, (A) a light transmissible material, such as cellulose nitrate, glass, poly(vinyl chloride), plastics, nylon, methacrylic resin and polypropylene, is disclosed in Japanese Patent Laid-opened No. Sho 62-66861, (B) polypropylene fibers and ceramics are disclosed in Japanese Patent Laid-opened No. Hei 2-68190, and (C) glass, ceramics, nylon, acryl and polyester resins are disclosed in Japanese Patent Laid-opened No. Hei 5-309267.
However, among the materials as disclosed above, it is reported that the one comprising an organic material as its main component has the disadvantage that the organic material can be decomposed and deteriorated due to catalytic reaction caused by a photocatalyst contained in the said material, and the durability thereof has therefore been problematic (see Pumiaki OotaniO Kobunsi Kako No.42, vol.5, page 18 (1993); "Titanium dioxide", by Manabu Kiyono, published by Gihodo, page 165).
Further, even though the substrate is composed of an inorganic material, such as glass and ceramics, there must be some problems in the durability property of the substrate, such that, if an organic polymer resin is used as an adhesive for carrying a photocatalyst on the substrate, the photocatalytic activity may be lowered due to coverage of the surface of photocatalyst particles with such resin, and that the photocatalyst may be exfoliated from the substrate due to the cause of decomposition and deterioration of said organic polymer resin by virtue of its photocatalytic activity.
In order to avoid having such problems as described above, a method called spattering method whereby any organic materials do not remain (Japanese Patent Laid-opened No. Sho 60-044053), a method to coat and bake an organic titanate (Japanese Patent Laid-opened No. Sho 60-118236), a method to spray and bake titania sol (Japanese Patent Laid-opened No. Hei 5-253544), and the others have been employed, in case that the substrate used is an inorganic heat-resistant material.
However, these methods have a problem that they require a process of baking the substrate at a high temperature in order to obtain production and crystalization of photocatalyst particles over the substrate and adhesive property with the substrate, and therefore, it is difficult to carry photocatalyst over a wide area and the production according to these methods requires very high cost.
Whereas, for carrying a photocatalyst onto a glass fiber paper, a method to use a metal oxide sol as an adhesive has been proposed (see Japanese Patent Laid-opened No. Hei 5-309267).
However, the adhesive property of a metal oxide sol, such as silica sol, is very weak because it is derived from van der Waars force (see Fine Ceramics, vol.1, page 216-223, 1980) so that the binding strength and durability of the adhesive were insufficient. Further, the method additionally requires a process of baking at a high temperature, and therefore, it was not applicable for all types of substrates including commonly used types of resins which are easily decomposed by heating.
In an example wherein a metal oxide, such as silica gel and clay mineral, in a sol state carrying photocatalyst powder thereon, there is a report that the photocatalytic decomposition reaction of propione aldehyde gas is accelerated by virtue of the effect of a substrate as an adsorbent (see Symposium, "Recent development in Photocatalytic Reaction", previous manuscripts, by Society for the Study of Photofunctional Materials, No.2-11, page 39, 1994).
However, no report has been made up till now describing that a substrate having excellent adhesive property and durability while keeping the high photocatalytic activity of a photocatalyst which is uniformly distributed in a metal oxide sol as described above is obtained.
Whereas, a method to fix a photocatalyst by using a fluororesin has been also proposed (see Japanese Patent Laid-opened No. Hei 6-315614). However, the price of fluororesin is high, and it is required to cover the most of the surface of photocatalyst particles with fluororesin in order to stick photocatalyst particles strongly. Accordingly, the catalytic activity of a photocatalyst becomes lower than the activity given by the same photocatalyst in powder form. Although an example that intends to carry a photocatalyst onto a substrate by means of mixing the photocatalyst with a binder resistant to decomposition, such as fluororesins and poly(organosiloxane), has been known (see EP-0633064), it is not sufficient to solve practically such problems as to adhesive property and durability.
As described above, the following three points can be given as problems to be solved when carrying a photocatalyst onto a substrate, which are (1) an adhesive property between a photocatalyst and a substrate should be good, (2) the photocatalytic activity of a photocatalyst does not degrade when it is carried onto a substrate, and (3) both of a substrate and an adhesive should not be deteriorated due to presence of a photocatalyst carried thereon and the substrate can keep its binding strength, durability and catalytic activity. Furthermore, when using a photocatalyst-carrying structure under a condition of a high temperature and high humidity, a property to maintain excellent adhesive property after dipping it into boiling water is required for the structure, for example.
Whereas, it is required for a photocatalyst coating material used for carrying a photocatalyst onto a substrate a property that the photocatalyst coating material causes neither its viscosity increase nor its particle sedimentation even after the preservation for at least one month and preferably more than three months. Also, a property that enables to carry a photocatalyst onto a substrate without deteriorating its photocatalytic activity when coating the photocatalyst onto a product for practical use, is required as well.
The inventors of the present invention have found a method to strongly glue a photocatalyst onto a substrate by providing a specific adhesive layer in between a photocatalyst layer and a substrate to thereby protect the substrate provided under the adhesive layer from its deterioration due to photocatalytic action derived from the photocatalyst and strongly glue the photocatalyst layer to the substrate and by making the adhesive layer resistant to deterioration due to photocatalytic action, providing a solution for the problems as described above.